Hanging after upgrading to jupyterlab-2.1.5

We use an environment where we add beakerx on top of docker-stacks:datascience-notebook

I recently upgraded this to the version that used jupyterlab 2.1.5 (most recent in docker hub) and now when the container running beakerx starts, it "hangs" -- looking at the processes within the container, the only processes are python/jupyterlab.

Does beakerx depend on jupyterlab 1.x?

The Dockerfile we've used is below:

```
USER root

RUN conda install --no-update-deps -c conda-forge --freeze-installed \
beakerx>1.3.0 && \
conda clean -afy && \
jupyter labextension install beakerx-jupyterlab

#
# Declutter
#
RUN echo "y" | /opt/conda/bin/jupyter-kernelspec remove -y \
clojure groovy kotlin sql

RUN rm -rf /home/jovyan && \
mkdir /home/jovyan && \
chown $NB_UID:$NB_GID /home/jovyan && \
rm -rf /usr/local/bin/fix-permissions

USER $NB_UID
```